This invention relates to toner dispensers for electrostatic copiers and duplicators and more particularly to detectors for warning a machine operator of impending exhaustion of the toner supply.
Conventional electrostatic copiers and duplicators use an electroscopic toner to develop latent electrostatic images from which copies are made. This toner is consumed in the copying process thereby periodically depleting the toner supply. When the supply of toner finally nears exhaustion, there may be a perceptible reduction in the density of the developed image and therefore, a corresponding degradation in copy quality. This latter condition is usually the first indication that the toner supply is in need of replenishment. A knowledgeable operator will, of course, add additional toner upon noticing that the density of the copied image has decreased, but several substandard copies may be made before that happens.
Accordingly, it has been recognized that an earlier warning of impending toner exhaustion would be desirable. One approach to solving that problem has been to use a transparent toner dispenser so that light emitted by a lamp at one side of the dispenser is visible at the other side when the toner supply drops below a predetermined level. Alternately, a light source on one side of the dispenser may be located to illuminate a photodetector on the opposite side of the dispenser to provide an indicating signal upon a preselected depletion in the toner level. This latter arrangement has been undesirable since the photocell housing may become coated with toner powder and will therefore not be activated properly upon the drop in toner below the preselected level.
The present invention is directed to the latter type of system in which both the lamp and the photocell are separated from the toner supply by transparent windows and a provision for periodically cleaning or wiping these windows is provided.